1. Field of the Invention
The present invention relates to an apparatus for controlling the air-fuel ratio of an internal combustion engine, and more particularly to an air-fuel ratio control apparatus which is capable of evaluating a deteriorated state of a catalytic converter for purifying an exhaust gas.
2. Description of the Related Art
There have been known various techniques for controlling the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine in order to achieve an appropriate purifying capability of a catalytic converter for purifying exhaust gases, such as a three-way catalytic converter, disposed in the exhaust passage of the internal combustion engine. See, for example, Japanese laid-open patent publication No. 9-324681 or U.S. Pat. No. 5,852,930, and Japanese laid-open patent publication No. 11-153051 or U.S. Pat. No. 6,112,517. According to the disclosed systems, an O2 sensor (oxygen concentration sensor) is disposed downstream of the catalytic converter, and the air-fuel ratio of an air-fuel mixture is controlled according to a feedback control process (specifically, a sliding mode control process) in order to converge the output of the O2 sensor to a predetermined target value (constant value).
The catalytic converter is progressively deteriorated and its purifying capability is lowered as the internal combustion engine is used over a long period of time. Therefore, the catalytic converter needs to be replaced with a brand-new catalytic converter when it has been deteriorated to a certain extent. It has thus been desired in the art to be able to adequately evaluate a deteriorated state of a catalytic converter which is combined with an internal combustion engine. Known systems for evaluating a deteriorated state of a catalytic converter are disclosed in Japanese patent No. 2526640 and Japanese laid-open patent publication No. 7-19033, for example. According to these disclosed evaluating systems, when the air-fuel ratio of an air-fuel mixture to be combusted by an internal combustion engine is changed from a leaner value to a richer value or from a richer value to a leaner value, the time required until the output of an O2 sensor that is positioned downstream of the catalytic converter is inverted and the period in which the output of the O2 sensor is inserted are measured. The deteriorated state of the catalytic converter is evaluated based on the evaluated values.
The disclosed systems, however, make it difficult to achieve an appropriate purifying capability of the catalytic converter at the time its deteriorated state is evaluated because it is necessary to positively change the air-fuel ratio in order to evaluate the deteriorated state of the catalytic converter. Stated otherwise, it is difficult for the disclosed system to evaluate the deteriorated state of the catalytic converter while at the same time maintaining an appropriate purifying capability of the catalytic converter.
The applicant of the present application has made an effort to develop an apparatus capable of evaluating a deteriorated state of a catalytic converter while controlling an air-fuel ratio to converge the output of an O2 sensor that is positioned downstream of the catalytic converter to a given target value, i.e., maintaining a good purifying capability of the catalytic converter. For example, reference should be made to Japanese patent application No. 2000-139860 (Japanese laid-open patent publication No. 2000-241349). The disclosed system is based on the fact that the value of a certain parameter which is determined from the data of the output of an O2 sensor that is positioned downstream of the catalytic converter exhibits characteristic properties as the deterioration of the catalytic converter progresses, and evaluates the deteriorated state of the catalytic converter based on the value of the parameter. The system thus arranged makes it possible to evaluate the deteriorated state of the catalytic converter while maintaining a good purifying capability of the catalytic converter.
In the above arrangements for controlling the air-fuel ratio to converge the output of the O2 sensor that is positioned downstream of the catalytic converter to a given target value (see Japanese laid-open patent publication No. 9-324681 or U.S. Pat. No. 5,852,930, and Japanese laid-open patent publication No. 11-153051 or U.S. Pat. No. 6,112,517), the target value for the output of the O2 sensor positioned downstream of the catalytic converter is basically a certain constant value. However, depending on the type of the catalytic converter, it may occasionally preferable to make variable the target value for the output of the O2 sensor depending on the operating state of the internal combustion engine for the purpose of enabling the catalytic converter to achieve an appropriate purifying capability.
However, a control system for controlling the air-fuel ratio according to the variable target value for the output of the O2 sensor positioned downstream of the catalytic converter tends to suffer certain drawbacks, given below, with respect to the reliability of the evaluation of the deteriorate state of the catalytic converter at the time of evaluating the deteriorate state of the catalytic converter while at the same time controlling the air-fuel ratio.
Since the output of the O2 sensor is nonlinear with respect to the oxygen concentration (see the solid-line curve a in FIG. 2 of the accompanying drawings), if the target value for the output of the O2 sensor changes, then the output of the O2 sensor changes differently when it is converged to the target value. As a result, the value of the parameter that is uniquely determined from the data of the output of the O2 sensor is affected by a factor other than the deteriorate state of the catalytic converter, and the deteriorate state of the catalytic converter which is evaluated based on the value of the parameter tends to become less reliable than expected.
It is therefore an object of the present invention to provide an apparatus for controlling the air-fuel ratio of an internal combustion engine to converge the output of an O2 sensor that is positioned downstream of a catalytic converter to a target value which is variably established depending on the operating state of the internal combustion engine, the apparatus being capable of appropriately evaluating the deteriorated state of the catalytic converter while compensating for adverse effects which are produced by making the target value variable.
To achieve the above object, there is provided in accordance with a first aspect of the present invention an apparatus for controlling the air-fuel ratio of an internal combustion engine, comprising an oxygen concentration sensor disposed downstream of a catalytic converter which is disposed in an exhaust passage of an internal combustion engine, air-fuel ratio manipulating means for manipulating the air-fuel ratio of an air-fuel mixture to be combusted in the internal combustion engine to converge an output of the oxygen concentration sensor to a target value established depending on an operating state of the internal combustion engine, parameter generating means for generating a deterioration evaluating parameter for evaluating a deteriorated state of the catalytic converter according to an algorithm predetermined from data of the output of the oxygen concentration sensor while the air-fuel ratio manipulating means is manipulating the air-fuel ratio of the air-fuel mixture, and deteriorated state evaluating means for correcting the deterioration evaluating parameter depending on an average value of the target value or the output of the oxygen concentration sensor, and evaluating the deteriorated state of the catalytic converter based on the corrected deterioration evaluating parameter.
According to a second aspect of the present invention, there is also provided an apparatus for controlling the air-fuel ratio of an internal combustion engine, comprising an oxygen concentration sensor disposed downstream of a catalytic converter which is disposed in an exhaust passage of an internal combustion engine, air-fuel ratio manipulating means for manipulating the air-fuel ratio of an air-fuel mixture to be combusted in the internal combustion engine to converge an output of the oxygen concentration sensor to a target value established depending on an operating state of the internal combustion engine, parameter generating means for generating a deterioration evaluating parameter for evaluating a deteriorated state of the catalytic converter according to an algorithm predetermined from data of the output of the oxygen concentration sensor while the air-fuel ratio manipulating means is manipulating the air-fuel ratio of the air-fuel mixture, and deteriorated state evaluating means for evaluating the deteriorated state of the catalytic converter by comparing the deterioration evaluating parameter with a predetermined decision value, the deteriorated state evaluating means comprising means for establishing the decision value depending on an average value of the target value or the output of the oxygen concentration sensor, and means for evaluating the deteriorated state of the catalytic converter by comparing the established decision value with the deterioration evaluating parameter.
With the above arrangement, the deterioration evaluating parameter generated from the data of the output of the oxygen concentration sensor is corrected according to the first aspect or the decision value to be compared with the deterioration evaluating parameter is established according to the second aspect, depending on the average value of the target value or the output of the oxygen concentration sensor, or stated otherwise depending on what level the output of the oxygen concentration sensor is controlled at by the air-fuel ratio manipulating means. In this manner, even if the target value for the output of the oxygen concentration sensor is established variably depending on the operating state of the internal combustion engine, the deteriorated state of the catalytic converter can be evaluated irrespectively of the target value.
Specifically, according to the first aspect of the present invention, the deterioration evaluating parameter is corrected depending on the average value of the target value or the output of the oxygen concentration sensor, so that the corrected deterioration evaluating parameter varies depending on only the deteriorated state of the catalytic converter, not the target value for the output of the oxygen concentration sensor. As a result, the deteriorated state of the catalytic converter can be evaluated appropriately based on the corrected deterioration evaluating parameter. According to the second aspect of the present invention, though the deterioration evaluating parameter is affected by the target value for the output of the oxygen concentration sensor, the decision value to be compared with the deterioration evaluating parameter is established depending on the average value of the target value or the output of the oxygen concentration sensor for thereby providing the same advantages as those of the first aspect. According to the present invention, because the deterioration evaluating parameter is generated from the data of the output of the oxygen concentration sensor while the air-fuel ratio manipulating means is manipulating the air-fuel ratio of the air-fuel mixture, i.e., the output of the oxygen concentration sensor is being converged to the target value, the deteriorated state of the catalytic converter can be evaluated highly reliably while the desired purifying performance of the catalytic converter is well maintained.
In the above apparatus, the deterioration evaluating parameter preferably comprises data representing a variation of a deterioration evaluating linear function whose variable components are represented by time-series data of the output of the oxygen concentration sensor.
Specifically, while the output of the oxygen concentration sensor is being converged to the target value, when the value of a suitable linear function, i.e., a function represented by a linear combination of time-series data of the output of the oxygen concentration sensor, whose variable components are represented by time-series data of the output of the oxygen concentration sensor, is determined from the time-series data of the output of the oxygen concentration sensor, the value of the linear function tends to be characteristically correlated to the degree of process of the deterioration of the catalytic converter. For example, when the catalytic converter is substantially brand-new, the value of the linear function is collected in the vicinity of a certain value. As the deterioration of the catalytic converter progresses, the value of the linear function tends to be spaced from the certain value. Therefore, the value of the linear function varies more greatly as the deterioration of the catalytic converter progresses.
By generating data representing the variation of the value of the linear function as the deterioration evaluating parameter, the correlation between the value of the deterioration evaluating parameter and the deteriorated state of the catalytic converter is increased, and hence the reliability of the evaluation of the deteriorated state of the catalytic converter is increased.
Though the deterioration evaluating parameter may be represented by the square of the difference between the value of the deterioration evaluating function and a certain value or the absolute value of the difference, the parameter generating means preferably comprises means for generating the deterioration evaluating parameter by effecting low-pass filtering on the square value or absolute value of the difference between values of the time-series data of the output of the oxygen concentration sensor and a predetermined value as a central value of the deterioration evaluating linear function.
The deterioration evaluating parameter determined by effecting low-pass filtering on the square value or absolute value of the difference is of an appropriate value representing the variation of the value of the deterioration evaluating linear function, and increases monotonously as the deterioration of the catalytic converter progresses. Therefore, the deteriorated state of the catalytic converter can be evaluated highly reliably based on the value of the deterioration evaluating parameter.
With the deterioration evaluating linear function being used to determine the deterioration evaluating parameter, the air-fuel ratio manipulating means preferably comprises means for sequentially generating a manipulated variable for manipulating the air-fuel ratio according to a sliding mode control process to converge the output of the oxygen concentration sensor to the target value, and manipulating the air-fuel ratio depending on the manipulated variable, the deterioration evaluating linear function comprising a linear function determined depending on a switching function used in the sliding mode control process.
Specifically, the above tendency of the value of the deterioration evaluating linear function depending on the deteriorated state of the catalytic converter is likely to manifest itself when the manipulated variable (e.g., a target air-fuel ratio) for manipulating the air-fuel ratio with the air-fuel ratio manipulating means is generated according to a sliding mode control process which is a feedback control process. In the case where the air-fuel ratio is manipulated according to the sliding mode control process, the deterioration evaluating linear function highly correlated to the deteriorated state of the catalytic converter is closely related to a switching function used in the sliding mode control process, and it is preferable to use a linear function determined depending on the switching function as the deterioration evaluating linear function.
More specifically, in the sliding mode control process, the linear function whose variable components are represented by time-series data of the difference between the output of the oxygen concentration sensor and the target value is used as the switching function. In the case where this switching function is used in the sliding mode control process, the deterioration evaluating linear function should preferably comprise a linear function where coefficient values of its variable components are identical to coefficient values of variable components of the switching function. The linear function may be the switching function itself which is used in the sliding mode control process.
By thus using the linear function determined depending on the switching function for the sliding mode control process as the deterioration evaluating linear function, the correlation between the value of the deterioration evaluating linear function and the deteriorated state of the catalytic converter manifests itself, allowing the deteriorated state of the catalytic converter to be evaluated properly based on the value of the deterioration evaluating linear function.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.